Enzyme reactions are used as a way to quantify glucose concentration. In a typical case, glucose oxidase (GOD) is used as the enzyme. GOD is an enzyme which is linked to flavin adenine dinucleotide (FAD), which is a coenzyme. The enzyme reaction of glucose when GOD is used proceeds according to the following chemical formula (In the formula, FADH2 is the reduction type of the FAD).Glucose+GOD/FAD→δ−Gluconolactone+GOD/FADH2 
When blood sugar levels are measured in a clinical setting, glucose concentrations are sometimes quantified by measuring the change in absorbance, which corresponds to the change in glucose concentration. However, the most common method is to measure the glucose concentration by amperometry. Amperometry is widely employed as a method for measuring glucose concentration in portable blood sugar measurement devices.
An example of how blood sugar is measured by amperometry is given below, for the case of measuring oxidation current. In the first step, a reaction system is constructed using blood, an enzyme, and an oxidative electron transfer medium (mediator). The result is that the above-mentioned enzyme reaction proceeds while a reductive mediator is produced by an oxidation-reduction reaction between the mediator and the FADH2 produced by this enzyme reaction. Potassium ferricyanide is commonly used as a mediator, in which case the reaction can be expressed by the following chemical formula.GOD/FADH2+2[Fe(CN)6]3−→GOD/FAD+2[Fe(CN)6]4−+2H+
Next, in the second step, voltage is applied to the reaction system using a pair of electrodes, which oxidizes the potassium ferrocyanide (releases electrons) and produces potassium ferricyanide as shown in the following chemical formula. The electrodes originating in the potassium ferrocyanide are supplied to the anode.[Fe(CN)6]4−→[Fe(CN)6]3−+e−
In the third step, the oxidation current value attributable to voltage application is measured, and the glucose concentration is computed on the basis of this measured value.
When blood sugar is measured using a portable blood sugar measurement device, a glucose sensor is used in which a reagent layer containing an enzyme and a mediator is formed between electrodes, and a reaction system is constituted between the electrodes by supplying blood to the reagent layer. This glucose sensor is installed in a portable blood sugar device, voltage is applied between the electrodes, the oxidation current value is measured, and the glucose concentration in the blood is quantified on the basis of this oxidation current value.
As discussed above, GOD is usually used as the enzyme, and potassium ferricyanide as the mediator. Nevertheless, in a reaction system combining GOD with potassium ferricyanide, the problems discussed below are encountered with a method for measuring glucose concentration by an electrochemical process, typified by amperometry.
The first of these problems is the effect of reductive substances. For instance, if we consider the measurement of glucose concentration in blood, there are reductive substances (such as ascorbic acid, glutathione, and Fe(II)2+) coexisted in the blood in addition to glucose. If a reductive substance other than potassium cyanide is present when voltage is applied to the reaction system, electrons originating in the oxidation of the reductive substance caused by voltage application will be supplied to the electrodes in addition to the electrons originating in the potassium ferrocyanide. As a result, the measured current value will include background current (noise) attributable to the electron transfer of the reductive substance. Accordingly, the measured glucose concentration will end up being greater than the actual glucose concentration. The greater is the amount of voltage applied between the electrodes, the more types and quantity of reductive substances that are oxidized, and the more pronounced is this measurement error. Therefore, when potassium ferricyanide is used as the mediator, blood sugar cannot be measured accurately unless the final concentration is determined by correcting the measured value. This effect of reductive substances is not limited to when blood sugar is measured, and is similarly encountered with other components when the concentration is computed on the basis of the oxidation current value.
Another problem pertains to the storage stability of the glucose sensor when glucose concentration is measured with a portable blood sugar measurement device and a glucose sensor. Potassium ferricyanide is susceptible to the effects of light and water, and when exposed to these, receives electrons from sources other than glucose and turns into reductive potassium ferrocyanide. If this happens, then the reaction system will contain both potassium ferrocyanide that has been rendered reductive by enzyme reaction, and potassium ferrocyanide that has been rendered reductive by exposure. As a result, just as with the reductive substance problem described above, the oxidation current during voltage application includes background current originating in the potassium ferrocyanide resulting from exposure. Consequently, the measured glucose concentration ends up being greater than the actual glucose concentration. To minimize this problem, the glucose sensor must be sealed in a pouch made from a light-blocking material, for example, so that the reagent layer in the glucose sensor is not exposed. Furthermore, to extend the service life of the glucose sensor, it has to be sealed in a moisture-tight state by performing nitrogen replacement or other such treatment in order to avoid exposure to moisture, and this complicates manufacture and drives up the cost when the glucose sensor is mass-produced on an industrial scale.